This is because amongst the easiest courses to qualify for in universities in Singapore nowadays is Engineering, even with A-level grades such as CCD. However, all engineering courses require at least a pass in O-level Physics. Thus it is unwise not to do O-level Physics. Some recent A-level grads have found out that that their poor grades qualify for only engineering courses, but they didn’t take Physics at either O or A level, thus finding themselves unable to proceed.
Some secondary schools emphasize doing Chemistry, instead of Physics, with the reason that Chemistry is a pre-requisite for Medicine or the Life Sciences, but they forget that the eventual A-level grades of most students will qualify them for Engineering but not Medicine or Dentistry.